Why are major telescopes always built in the middle of nowhere, why not on large sky scrapers or university campuses?

This is a good question. I agree that it would be more convenient to have all our telescopes on the roof of the astronomy building, here on campus rather than having to fly all around to go observing (though perhaps not as much fun!).

There are two good reasons why telescopes are generally built "in the middle of nowhere":

1- Light pollution. Where there are people, there is light. And this light can interfere with astronomical observations. If you have tried to look at the sky during the night in a big city, you will have noticed that you are only able to see a handful of stars, even on a cloudless night. The glow of the lights makes the sky look bright and makes impossible good astronomical observations. For more details on light pollution, see this previously answered question, and have a look at this map of Earth, showing the regions affected by light pollution.

A map of the world, showing the extent of light pollution. The brighter a region is, the worse light pollution is for astronomical observations. Credit: P. Cinzano, F. Falchi (University of Padova), C. D. Elvidge (NOAA National Geophysical Data Center, Boulder). Copyright Royal Astronomical Society. Reproduced from the Monthly Notices of the RAS by permission of Blackwell Science.

The usual form of light pollution we are used to means that optical telescopes have a hard time seeing but radio telescopes suffer the same problems. Cell phones, wireless internet, GPS satellites, and even planes and cars can all be "seen" by radio telescopes. They also have to be built in remote locations away from transmitters or radio sources so that they can observe celestial radio waves directly.

2- Atmospheric conditions. We use space telescopes, like Hubble, because you gain a lot by getting rid of the atmosphere. For groundbased telescopes, the light from astronomical objects that they receive has to go through all of the atmosphere, which causes attenuation and distortion. Therefore, the less atmosphere and the more stable the atmosphere, the better. For some types of telescopes, humidity is also a problem, so the dryer the atmosphere the better. The sites in the world that fit these criteria are few and generally remote: the top of volcano Mauna Kea in Hawaii, the Atacama desert in Northern Chile (and other sites in altitude in Chile), Antarctica, the desert in Arizona, California and New Mexico (though this last places suffer more and more from light pollution as cities get bigger and bigger).

For a combination of all these reasons, astronomers end up having to travel around the world to visit telescopes that are located in prime locations. Though these days, it is getting more and more common for observations to be done remotely, thanks to the Internet. Some telescopes are now setup in a way that allows astronomers to control them by sending commands through the Internet, which requires only a telescope operator to be present on site. For example, while I am writing this answer to you, I am observing galaxies from the comfort of my office in Ithaca, NY, using the Arecibo telescope, a radio telescope located in Puerto Rico!

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